Confirmed: It’s possible to be too clean

A study released in June added to the growing mound of evidence that the too-clean-for-health hypothesis might be on track. That study, published in the Journal of Allergy and Clinical Immunology, found that Amish children who were raised on farms were less likely to develop allergies and asthma than their peers.

Why would exposure to dirt and microbes make a kid less sensitive to pollen and the like?

For one thing, it’s exposure to pathogens that allows the immune system to become fine-tuned as it learns to differentiate between harmful and harmless irritants.